Could please shed some light on 3rd person imperatives? I don't exactly know the meaning of it, and its difference to the subjunctive. Could someone also give me a paradigm on its formation (including the rare passive forms), please?

As you can see, it is an incomplete paradigm, and indeed it was not commonly used during the Classical period. In many cases, to form the passive, you simply add -R to the active form, just as many indicative passives end in R.

The difference between a jussive/hortatory/volitive subjunctive and an imperative is that the subjunctive expresses a wish on the speaker's behalf and an imperative issues a command. A so called 'subjunctive command' is less powerful and much less intrusive or wilful than an imperative. If a Roman were to use a subjunctive, such as Venias, it would have a meaning more like "Would that you might come" or "Please come" or "Come on!". To say Veni means, quite bluntly, "Come!".

The third person poses somewhat of a problem, because it seems very foreign to us. How can someone issue an order to the third person when the third person is by definition someone who isn't being spoken to? This imperative is much less direct than the other, in that the person is not spoken to, but the force of the command is still stronger than the subjunctive. Veniat means "let him come". Venito means "He is going to come (if I have anything to say about it)", or "He shall come!" implying that the speaker's wish is very strong. This is usually restricted to legal contexts, as are the rest of the future imperatives, except with a select few verbs which lack the present imperative (e.g. scio, memini).